The vestibular system is a portion of the inner ear that senses both angular and linear motion of the head, and encodes information representative of that motion as electrical signals. These signals serve as feedback control signals that the brain processes to maintain balance, determine an orientation with respect to surroundings, and cause the body to stand, walk, and perform other functions that require balance and stability.
When the vestibular system becomes damaged, the brain receives fewer feedback control signals, and the feedback control signals that are received may contain incorrect information. A lack of adequate feedback and/or the receipt of incorrect feedback can impair balance and lead to conditions such as vertigo, unsteadiness, dizziness, blurred vision, reduced ability to stand or walk, and cognitive problems. The intensity of these symptoms can range from mild to debilitating. A serious consequence of impaired vestibular function, especially among the elderly, is an increased risk of falling.
Vestibular prostheses have been developed to treat absent or impaired vestibular function by replacing or bypassing damaged vestibular structures in the inner ear. The goal of a vestibular prosthesis is to detect head motion in different directions and, in response to the motion, produce electrical signals that stimulate the vestibular nerve in a way that mimics how the patient's own vestibular system, if functioning properly, would normally stimulate the vestibular nerve.
The anatomy of the vestibular system defines the kind of information that is carried to the vestibular nerve. The vestibular system includes five different organs: two obelisk-shaped organs that sense acceleration and gravity, and three vestibular canals that sense head rotation. The three vestibular canals are nearly orthogonal and define the axes of a head-fixed coordinate system. The vestibular canals provide a measurement of all three dimensions of head rotation as signals that each correspond to rotation about one of the head-fixed axes (i.e., the vestibular canals).
For the signals produced by the vestibular prosthesis to be correctly interpreted by the vestibular nerve, the signals must correspond to rotation about the head-fixed axes, which may be nearly orthogonal. Conventional approaches for appropriately configuring the signals include physically aligning the motion sensors with the vestibular canals during surgery. To obtain a required level of accuracy, these approaches often require painstaking and prolonged surgery. Typically, prolonging the duration of a surgery increases the risk of complication and lengthens the time for recovery.
Other conventional alignment approaches avoid physical alignment of the sensors and instead perform corrections on the sensor signals to make them correspond to rotation about the head-fixed axes before they are delivered to the vestibular nerve. The vestibular prostheses using these approaches generally perform the signal corrections in digital hardware using a variety of digital signal processing (DSP) techniques. For example, such a prosthesis might include an analog-to-digital converter for digitizing each of the motion signals and a microcontroller or digital signal processor for processing the signals via digital floating-point arithmetic. Power consumption associated with digital techniques can be substantial and the speed with which the digital calculations can be performed is limited by clock speed. Increasing the speed of digital processing requires increasing the clock rate, which in turn, requires greater power consumption. Power consumption is an important factor because it determines the life of the prosthesis and the size of the battery that powers the vestibular prosthesis.